Scientists will soon learn if looks indeed are deceiving. A NASA
spacecraft called MESSENGER is expected to complete a circuitous,
six-year voyage on Thursday and put itself into orbit around
Mercury for a year-long study, becoming the innermost planet's
first artificial satellite.

Circling as close as 29 million miles from the sun --
compared to Earth's 93 million -- Mercury feels the brunt of the
sun's heat and its muscular gravitational grasp. The planet
completes an orbit around the sun in 88 days, compared to the 365
days it takes Earth. Oddly though, Mercury rotates so slowly
that 176 days on Earth will pass between a single sunrise and
sunset. On Mercury time, two years go by every day.

With only a tenuous atmosphere, Mercury's skies are glaringly
bright in the direction of the sun, which looms three times
larger than it appears on Earth. Away from sun, it is black as
night. And it is the only planet in the solar system that
shows signs of shrinkage.

Curving lines of cliffs, known as scarps, stretch for hundreds of
miles over most of the planet's surface, mini-mountain ranges
that speak to a time when a hotter, slightly larger Mercury
cooled and shrank.

"Where the scarps cut into pre-existing impact craters, the
craters have either become half, or squeezed horizontally,"
MESSENGER lead scientist Sean Solomon, with the Carnegie
Institution of Washington, told Discovery News.

"The interpretation of the scarps is that they are the surface
expression of faults, somewhat like the one Earth experienced
with the magnitude-9 earthquake on Friday," he said.

On Earth, the faults stem from the huge horizontal motions of
tectonic plates inside the planet. They are constantly grinding
against one another, though for most of the time, remain locked.
When they suddenly release, such as what happened off the coast
of Japan on Friday, an earthquake occurs, releasing energy that's
been stored up over years or decades.

"The Mercury scarps are smaller scale examples of those kind of
fault motions, though they are not tied to plate tectonics, but
to the horizontal shortening of the crust almost everywhere we
look," Solomon said.

The amount of contraction, however, is not nearly enough to
explain Mercury's great density. Scientists think Mercury
contains a substantially higher percentage of iron than Earth,
Venus and Mars, the solar system's other rocky planets.

Most of Earth's iron resides in its core, which comprises about
30 percent of the planet's mass. Mercury's iron core, by
comparison, is believed to be about 60 percent of the planet's
mass.

Mercury may have started off with a different blend of materials
when the solar system was forming 4.5 billion years ago. Or
perhaps it had the same mix, but lost its lighter elements due to
extreme heating, or after a massive impact that blasted a huge
chunk of Mercury's developing body into space.

"The mystery is how do you assemble a planet that ends up almost
two-thirds metal and one-third rock instead of the more common
two-thirds rock and one-third metal?" Solomon said.

Over the next year, scientists hope MESSENGER will relay some
answers.

After two sneak previews of Mercury, MESSENGER is scheduled to
fire its braking rockets for 15 minutes Thursday night and settle
into an orbit that will come as close as 120 miles above the
planet's surface. In addition to studying the planet's surface
composition, instruments will probe Mercury's magnetic field and
look for water ice in permanently shadowed craters near its
poles.

"This really is like a whole new mission because we are going to
be in orbit," said Andy Calloway, the mission operations manager
at Johns Hopkins University Applied Physics Laboratory in
Maryland. "We think we're ready to go and are looking forward to
it."